Silicone rubber is one of the most versatile standard mold-making materials. Flexible yet durable and capable of reproducing high levels of detail, silicone rubber is prized for use in making molds of hard items. Like all rubbers, silicone can rip or break. But if your mold is thick and breaks cleanly into two pieces, you can use a silicone-based glue to restore it, making it as good as new and ready for the next molding project.
Clean both of the broken halves of the mold with hand soap and water and dry with a lint-free cloth; this will keep any dust or grit from interfering with the seal or the shape of the repaired mold.
Apply a light layer of silicone contact adhesive to each side of the broken mold. Spread the adhesive so that it completely covers the edges of the silicone mold pieces where they were broken or torn. Use a damp rag to wipe away any glue that spreads or leaks onto other areas of the mold.
Let the adhesive dry. Read the instructions on the bottle to get the right drying times for the adhesive you're using and follow them to the letter. Different formulas of contact adhesive will have different drying times, and it's important to get the timing just right; too little or too much time will prevent the adhesive from adhering correctly.
Line up the halves of the mold to reattach them. Do not let them touch (the glue will bond instantly when you do), but hold them as close as possible without making contact so that you can make sure the edges will line up perfectly.
Press the pieces of the mold together. Start with the bottom of the two edges, holding the pieces at an angle so that only the bottoms connect. Line them up as perfectly as possible before pressing them to touch, then slowly roll the two pieces together bottom to tip, watching the edges as you go to make sure they continue to line up. Once the two sides have been pressed together, the glue will bond permanently. Use the mold after the glue manufacturer's recommended wait time for setting.